What is one of the dumbest things a teacher/prof has taught you or told you that was completely incorrect?

I had a 7th grade teacher tell us Saddam was the antichrist and the world was ending. On parent's night she even said it was a waste of time because we wouldn't make it to high school, the world would be over by then. She didn't last long. That and Christopher Columbus discovering America were my top 2.

I am a new science teacher and I've told my students many incorrect things unfortunately.

Being a teacher and having to answer random science questions has taught me one thing - teachers are full of misinformation! (I assume everyone else is too of course).

My worst blunder was when talking about osmosis during student teaching I was inevitably asked what would happen if you drank distilled water. I said that it could kill you because it would force important nutrients out of your cells and into the hypotonic fluid surrounding the cells. All of this made sense to me at the time and I am pretty sure one of my college professors told me something like this at one point. Well, it was only a few weeks later that I noticed distilled water being sold for drinking purposes at the grocery store. I did some follow up research, and realized that I was an idiot. My assumption was that drinking some distilled water would expose your cells to pure distilled water when in reality, you consume plenty of other substances that will dissolve into the distilled water and make it quite harmless.

As embarrassing as this is ... I hope that as the years pass and I have more experience, I will make far fewer blunders and actually be able to use many of my mistakes to better educate my students.

My 7th grade social studies teacher told us that people in the US were upset about the Korean War because we had just gotten out of World War II, which had lasted 8 years for the US. That one has always stuck with me as being unbelievably dumb